For each FASTA record, the first "word" on the definition line is used as the name of the produced datum object. The entire line is assigned to:

$datum->set_generic( 'fasta_def_line' => $line )

So you can retrieve it by calling:

my $line = $datum->get_generic('fasta_def_line');

BioPerl actually parses definition lines to get GIs and such out of there, so if you're looking for that, use Bio::SeqIO from the bioperl-live distribution. You can always pass the resulting Bio::Seq objects to Bio::Phylo::Matrices::Datum->new_from_bioperl to turn the Bio::Seq objects that Bio::SeqIO produces into Bio::Phylo::Matrices::Datum objects.